Perhaps she wasn't fully aware of her rights — or the airline's — but the tweetstorm that unfolded reminds us why it's always important to know your rights as a passenger.

When you purchase an airline seat, you are required to acknowledge that you agree to the contract of carriage, a legally binding agreement between you and the airline, usually by checking a box next to the phrase "I agree to the terms and conditions of the contract of carriage" or some variation thereof.

Delta's contract of carriage states: "Delta may substitute alternate carriers or aircraft, delay or cancel flights, change seat assignments, and alter or omit stopping places shown on the ticket at any time. Schedules are subject to change without notice."

When you buy a plane ticket, the airline is only obligated to get you from point A to point B. Everything else that comes with it (reserved seating, overhead bin space, baggage allowance, food and beverage service), comes at the discretion of the airline. Because you're not buying an actual seat — you're purchasing transportation.

When you're asked by a flight attendant or gate agent to change seats, it's usually to help families sit together, allow caregivers to sit next to patients, or to accommodate an air marshal or other airline employee. They also might ask you to move for safety reasons or to help redistribute the weight balance of the aircraft, especially on smaller planes.

If it happens to you, respond politely and graciously. Over the years, I've been asked countless times to move seats and have done so without complaining. Nearly every time, the flight attendant has thanked my willingness to be flexible with free bottles of wine or a snack.

Your Boarding Pass Is Your Assigned Seat

If a gate agent comes onboard and hands you a new boarding pass, that is the seat you are required to sit in, regardless of how much you paid for your ticket or the class of service you purchased. The gate agent has supreme power of seating assignments for every flight and dictates who sits where. If he or she asks you to move, do it.

You May Be Entitled to a Refund

If you're downgraded from a seat with extra legroom to a regular economy seat, the airline will reimburse you the difference, such as in Coulter's case, where Delta refunded her the $30 extra fee she paid for the Delta Comfort+ seat that was given to another passenger.

The contract of carriage for American Airlines, as another example, states that you can request a refund for a variety of reasons, including getting re-booked on a flight that causes you to miss a connection, or if you get moved from a "preferred aisle/window seat to a preferred middle seat." Those "preferred" seats refer to the coveted seats near the front of the economy cabin or in emergency exit rows that typically cost extra.

If your seat gets moved and you feel you are owed a refund, contact the airline's customer service department to request one.